1. Field of the Invention
The present invention relates to dual energy radiography and more particularly to an automatic, calibration free method for dual energy material decomposition in digital radiography.
2. Description of the Related Art
Dual energy radiography is a technique that involves taking two radiographic images of an object at different radiation energy. A high energy image and a low energy image are combined to form images which highlight different material compositions in the object. Typically in medical imaging applications, bone or soft-tissue images are reconstructed.
There are different types of algorithms to obtain material decomposition images in dual energy radiography.
A first category of algorithms decomposes the high and low energy images into equivalent thicknesses of basis materials. These methods represent the attenuation coefficient as a weighted sum of non-linear basis functions with the weights representing the basis material thicknesses. This technique requires an extensive calibration procedure whereby the high and low energy pixel values are measured for different kVp pairs using a calibration phantom containing a range of thicknesses for both basis materials, usually aluminum and Lucite.
A second category of algorithms compute the decomposition images as a linear weighted log-subtraction of the low and high energy images.
In this straightforward approach it is assumed that images are generated using mono-energetic spectra and that the imaged object consists of 2 materials, bone and soft tissue. The signal at pixel (x,y) in the low and high energy images is given as follows:PL˜Exp(−μS,LzS−μB,LzB)PH˜Exp(−μS,HzS−μB,HzB)
With μS,L and μB,L the attenuation coefficients of soft tissue and bone for the low-energy spectrum, μS,H and μB,H the attenuation coefficients of soft tissue and bone for the high-energy spectrum and zS and zB the thicknesses of soft tissue and bone in the patient for pixel position (x, y).
By computing the subtraction of the log-converted images with a weight parameter w, one can generate material specific images.
                              Log          ⁡                      (                          P              OUT                        )                          =                ⁢                              Log            ⁡                          (                              P                H                            )                                -                      w            ⁢                                                  ⁢                          Log              ⁡                              (                                  P                  L                                )                                                                            =                ⁢                                            -                              μ                                  S                  ,                  H                                                      ⁢                          z              S                                -                                    μ                              B                ,                H                                      ⁢                          z              B                                -                      w            ⁡                          (                                                                    -                                          μ                                              S                        ,                        L                                                                              ⁢                                      z                    S                                                  -                                                      μ                                          B                      ,                      L                                                        ⁢                                      z                    B                                                              )                                                              =                ⁢                                            -                              (                                                      μ                                          S                      ,                      H                                                        -                                      W                    ⁢                                                                                  ⁢                                          μ                                              S                        ,                        L                                                                                            )                                      ⁢                          z              S                                -                                    (                                                μ                                      B                    ,                    H                                                  -                                  W                  ⁢                                                                          ⁢                                      μ                                          B                      ,                      L                                                                                  )                        ⁢                          z              B                                          
If the weight parameter is defined as wS=μS,H/μS,L a bone image is generated. In a similar way a soft tissue image is generated if the weight parameter is defined as wB=μB,H/μB,L.
In prior art the weight parameters are manually selected and adjusted by the observer during image review.
In prior art patent WO 02/19909 A1 the weight parameters are automatically computed in an iterative way using cancellation metrics.